Freedom Tower design raises cables to new heights

February 01, 2004 |

Cables will be a prominent feature of the building that is to be constructed on the World Trade Center site in New York City. Freedom Tower will mark the first use at the top of a high-rise building of cables that function in a manner similar to their application on bridges.

Renderings of the building, being designed by the New York office of Skidmore, Owings & Merrill and based on a conceptual plan by World Trade Center site master planner Daniel Libeskind, reveal a tower with 70 occupied floors, topped by a double-masted broadcast antenna that will increase the structure's height to a symbolic 1,776 feet. The cables will stabilize the platform on which the antenna sits and provide structural redundancy for the tower itself.

The cable system will begin above the 1,100-foot level, where the building's rectilinear core transitions into two 500-foot-tall concrete cylinders. Cables will be attached at the top to a steel "crown" supported by these two core elements, and at the bottom to the main structure. In addition to providing a base for the antenna, the platform will provide a work surface for erecting the cables with a small winch.

The steel-framed building will have a concrete core. Tishman Construction, which also built the original Twin Towers, will be Freedom Tower's general contractor.

As much as 20% of the building's peak electrical requirements would be provided by wind turbines proposed for the top of the building. From 30 to 40 wind turbines are envisioned, depending on the final configuration of the top of the building.

The building's north and south elevations rise straight up from its base. The east and west elevations step back, or twist, along a diagonal that runs from northwest to southeast at the top. In addition to providing additional rigidity for the structure, this rotation will allow the wind turbines to face into prevailing winds.

Freedom Tower will share some structural similarities with Chicago's 1,450-foot John Hancock Center. But Freedom's structure will be interior to the exterior wall, although visible through it; the Hancock's structure is outside the curtain wall.

The Hancock has exterior framing members angled at 45 degrees, while some Freedom Tower members will slope at a much steeper angle of about 70 degrees.

Freedom Tower's design will incorporate a number of refinements prompted by the 9/11 terrorist attacks on the World Trade Center, including stairways that are 30% wider than required by code. For fire protection, two-thirds more water than required by code will be stored at the top of the building. All stairs will be enclosed by concrete shear walls, and on lower floors multiple egress paths will be provided to enable occupants to reach the building perimeter. Redundant risers will be available to supply fire sprinklers.

These features are "just the beginning of some of the ground we hope to be breaking" with Freedom Center," says Ken Lewis, SOM's project manager. The building is being designed to qualify, at minimum, for certification under the U.S. Green Building Council's Leadership in Environment and Design rating program.

Groundbreaking for Freedom Tower is scheduled for August, with the completion of its design expected about the end of this year.

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